dc motor circuits notes 2up

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Basic DC Motor Circuits

Living with the Lab

Gerald Recktenwald

Portland State University

[email protected]

LWTL: DC Motor

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DC Motor Learning Objectives

Explain the role of a snubber diode

Describe how PWM controls DC motor speed

Implement a transistor circuit and Arduino program for

PWM control of the DC motor

Use a potentiometer as input to a program that controls fan

speed

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What is a snubber diode

and why should I care?

LWTL: DC Motor

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Simplest DC Motor Circuit

Connect the motor to a DC power supply

+5V +5V

II

Switch open

Switch closed

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Current continues after switch is opened

Opening the switch does not immediately stop current in the motor windings.

+5V

II

+

Inductive behavior of themotor causes current tocontinue to flow when theswitch is opened suddenly.

Charge builds up on whatwas the negative terminalof the motor.

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Reverse current

Charge build-up can cause damage

+5V

I

+

Arc acrossthe switchand dischargeto ground

Reverse current surgethrough the voltage supply

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Motor Model

Simple model of a DC motor:

Windings have inductance and resistance

Inductor stores electrical energy in the windings

We need to provide a way to safely dissipate electrical energy when the switch is opened

+5V

I

+5V

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Flyback diode or snubber diode

Adding a diode in parallel with the motor provides a path for dissipation of stored energy when the switch is opened

+5VThe flyback diode allowscharge to dissipatewithout arcing acrossthe switch, or withoutflowing back to groundthrough the +5V voltagesupply.

+

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Pulse-width modulation (PWM)

for DC motor speed control

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Controlling DC Motor Speed

The voltage supplied to a DC motor controls its speed

Arduino cannot supply variable DC output

Arduino lacks a true analog output

Use Pulse-width modulation (PWM) to simulate a variable DC supply voltage

PWM is a common technique for supplying variable power levels to slow electrical devices such as resistive loads, LEDs, and DC motors

Arduino Uno has 6 PWM pins: Digital I/O pins 3, 5, 6, 9,10, and 11

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Arduno Uno has 6 PWM pins

Look for the ~ prefix on the digital pin label, e.g. ~3

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PWM: Pulsed with modulation

PWM simulates DC voltage control for slow loads

The effective voltage is

is called the duty cycle

...

o

c

Vs

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Arduino PWM commands

Configure the output pin:

Set the duty cycle

The duty cycle is an 8 bit value:

0 duty_cycle 255

PWM_pin = ... ; // one of 3, 5, 6, 9, 10, 11!!void setup() {! pinMode( PWM_pin, OUTPUT);!}!

void loop() {! int duty_cycle = 150; // between 0 and 255!! analogWrite( PWM_pin, duty_cycle );!}!

Using a transistor to switch the load

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Transistor as the switching device

Each Arduino output channels has a 40 mA limit

The maximum current draw for an Arduino is 200 mA

Use Arduino as the brain

Let another switching element be the brawn

LWTL: DC Motor

NPN General Pupose AmplifierThis device is designed for use as a medium power amplifier andswitch requiring collector currents up to 500 mA.

MMBT44012N4401

Absolute Maximum Ratings* TA = 25C unless otherwise noted

*These ratings are limiting values above which the serviceability of any semiconductor device may be impaired.NOTES:1) These ratings are based on a maximum junction temperature of 150 degrees C.2) These are steady state limits. The factory should be consulted on applications involving pulsed or low duty cycle operations.

Symbol Parameter Value UnitsVCEO Collector-Emitter Voltage 40 VVCBO Collector-Base Voltage 60 VVEBO Emitter-Base Voltage 6.0 VIC Collector Current - Continuous 600 mA

TJ, Tstg Operating and Storage Junction Temperature Range -55 to +150 C

Thermal Characteristics TA = 25C unless otherwise notedSymbol Characteristic Max Units

2N4401 *MMBT4401PD Total Device Dissipation

Derate above 25C6255.0

3502.8

mWmW/C

R!JC Thermal Resistance, Junction to Case 83.3 C/WR!JA Thermal Resistance, Junction to Ambient 200 357 C/W

C B ETO-92

C

B

E

SOT-23Mark: 2X

*Device mounted on FR-4 PCB 1.6" X 1.6" X 0.06."

" 2001 Fairchild Semiconductor Corporation

2N4401 / MM

BT4401

2N4401/MMBT4401, Rev A

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Use an NPN Transistor as a switch

This device is designed for use as a medium power amplifier and switch requiring collector currents up to 500 mA

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Electronic components in the fan kit

Transistor

Diode

220 or 330 resistor

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Replace the Switch with a Transistor

A transistor allows on/off control to be automated and it allows switching of more current than an Arduino digital pin can supply.

+5V

P2N2222Pin 9

330

1N4001diode

NPN transistor

Pin 9 or another PWM pin drives the transistor base

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Alternative locations for the transistor

Moving the transistor (and any switch) between the power supply and the motor adds a bit of safety by tying the motor to ground when the system is idle

+5V

PWMsignal

NPNtransistor

PNPtransistor

+5V

PWMsignal

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Diode and transistor orientation

PWMsignal

330

+5V

Collector: Connect to +5VBase: Connect to motorcontrol pin on ArduinoEmitter: Connect topositive terminal of motor

Orient the diode so that thesilver stripe is at the samevoltage as the positivemotor terminal

+

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Arduno Uno has 5 PWM pins

Look for the ~ prefix on the digital pin label, e.g. ~3

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DC Motor Circuit on tiny breadboard

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+5V connections

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PWM signal is connected to transistor base

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Arduino program to spin the DC Motor

// spin_DC_motor.ino Use PWM to control DC motor speed!!int motorPin = 3; // Pin 3 has PWM, connected it to the DC motor!!void setup() {! pinMode(motorPin, OUTPUT); // Set motor pin to output mode!}!!void loop() {! analogWrite(motorPin, 150); // Motor at 150/255 of full speed! delay(1000);! analogWrite(motorPin, 250); // Motor at 250/255 of full speed! delay(1000);!}!!

Code is in spin_DC_motor.ino

User input to control fan speed

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Adjust fan speed with potentiometer input

Use the potentiometer circuit from the earlier analog input exercise

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// File: DC_motor_speed_control.pde!//!// Use potentiometer input to set the speed of a DC motor!// Output to the motor is PWM!!int motorPin = 3; // pin connected to the DC motor!int potPin = 1; // analog input connected to the potentiometer!!void setup()!{! pinMode(motorPin, OUTPUT);!}!!void loop()!{! int PWMoutput, potReading;! ! potReading = analogRead(potPin);! PWMoutput = map(potReading, 0, 1023, 0, 255 );! analogWrite(motorPin, PWMoutput);!}!!

Code is in DC_motor_speed_control.ino

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Each time through the loop:

Read the voltage at the potentiometer wiper

Input value is a 10-bit integer: 0 potReading 1023

Scale the 10-bit value (max 1023) to an 8-bit value (max 255)

PWMoutput = map( potReading, 0, 1023, 0, 255 );!

Update the PWM signal

analogWrite(motorPin, PWMoutput);!

void loop() {!! int PWMoutput, potReading;!! potReading = analogRead(potPin);! PWMoutput = map(potReading, 0, 1023, 0, 255 );! analogWrite(motorPin, PWMoutput);!}!

range for

potReading

range for

PWMoutput

dc motor circuits notes 2up

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